Typically if there is no change on your system, every day you should see the same performance, day in and day out. However, I'm guessing something did change which is why you are seeing a change in performance. Think back to what has changed since you noticed the system has slowed and thats most likely the culprit. Usually the things that can cause issues are:
- did you patch your OS or add/remove any software?
- did you upgrade moodle?
- did you see an increase in utilization on your site?
- did you add any blocks to your site causing moodle to behave differently?
- have you implemented caching? and is it actually working as expected?